The schoolboy disappeared on 28 November 1994 and his body was found in woods near his Peterborough home the following day.
He had been strangled and his naked body deliberately posed in a star shape by his killer.
James Watson, now 41 but 13 at the time of Rikki’s death, was found guilty by jurors at the Old Bailey in London.
He was the second person to stand trial for Rikki’s murder, after the boy’s mother Ruth Neave was cleared of the killing by a jury in 1996.
Watson was convicted after jurors were given a majority verdict direction by judge Mrs Justice McGowan.
A cold case investigation into Rikki’s murder was opened in 2015.
Adhesive tapings from Rikki’s clothes were examined and a DNA match to Watson was made.
Watson claimed he may have lifted Rikki to help him see over a fence, but police found archive TV footage showing there was no fence at the time.
